Sydney, Jul 16 (IANS): Two different vaccine viruses, used simultaneously to control an acute respiratory disease occurring in chickens, have combined to produce new infectious viruses, says new research.

The vaccines were used to control infectious laryngotracheitis (ILT).

ILT can have up to 20 percent mortality rate in some flocks and has a significant economic impact in the poultry industry.

Researchers from the University of Melbourne found that when two different ILT vaccine strains were used in the same populations, they combined into two new strains (a process known as recombination), resulting in disease outbreaks, the journal Science reported.

Neither the ILT virus or the new strains can be transmitted to humans or other animals, and do not pose a food safety risk, said a university statement.

The study was led by Joanne Devlin, Glenn Browning, professor and Sang-Won Lee and colleagues at the Asia-Pacific Centre for Animal Health, University of Melbourne and NICTA's Victoria Research Lab.

Devlin said the combining of live vaccine virus strains outside of the lab was previously thought to be highly unlikely, but this study shows that it is possible and has led to disease outbreaks in poultry flocks.

"We alerted the Australian Pesticide and Veterinary Medicines Authority (APVMA) to our findings and they are now working closely with our research team, vaccine registrants and the poultry industry to determine both short and long term regulatory actions," she said.

The ILT vaccines are 'live attenuated vaccines', which means that the virus has some disease-causing factors removed but the immune system still recognises the virus to defend against a real infection.

"Live vaccines are used throughout the world to control ILT in poultry. For over 40 years the vaccines used in Australia were derived from an Australian virus strain. But following a vaccine shortage another vaccine originating from Europe was registered in 2006 and rapidly became widely used," Devlin said.

"Shortly after the introduction of the European strain of vaccine, two new strains of ILT virus were found to be responsible for most of the outbreaks of disease in New South Wales and Victoria. So we sought to examine the origin of these two new strains," Devlin added.